THE STEAM-ENGINE. 



565 



that the time of transit between terminus and terminus of a line of railway laid 

 down with gradients, varying from twenty to thirty feet a mile, will be practi- 

 cally the same as it would be on a line oif the same length constructed upon a 

 dead level ; and not only will the time of transport be equal, but the quantity 

 of moving power expended will not be materially different. The difference 

 between the circumstances of the transport in the two cases will be merely 

 that, on the undulating line, a varying velocity will be imparted to the train and 

 a varying resistance opposed to the moved power ; while on the level line the 

 train would be moved at a uniform speed, and the engine worked against a 

 uniform resistance. These conclusions have been abundantly confirmed by 

 the experiments made in last July with the Hecla engine above referred to. 

 The line of railway between Liverpool and Birmingham on which the experi- 

 ment was made extended over a distance of ninety-five miles, and the gradients 

 on which the effects were observed varied from a level to thirty feet per mile, 

 a great portion of the line being a dead level. The following table shows the 

 uniform speed with which the train ascended and descended the several 

 gradients, and also the mean of the ascent and descent in each case, as well 

 as the speed upon the level parts of the line : 



From this table it is apparent that the gradients do possess the compensating 

 power with respect to speed already mentioned. The discrepancies existing 

 among the mean values of the speed are only what may be fairly ascribed to 

 casual variations in the moving power. The experiment was made under 

 favorable circumstances : little disturbance was produced from the atmosphere ; 

 the day was quite calm. In the same experiment it was found that the water 

 evaporated varied very nearly in proportion to the varying resistance, and the 

 amount of that evaporation may be taken as affording an approximation to the 

 mean amount of resistance. Taking the trip to and from Birmingham, over 

 the distance of 190 miles, the mean evaporation per mile was 3.36 cubic feet 

 of water. The volume of steam produced by this quantity of water will be 

 determined approximately by calculating the number of revolutions of the 

 driving-wheels necessary to move the engine one mile. The driving-wheels 

 being 5 feet in diameter, their circumference was 15.7 feet, and consequently 

 in passing over a mile they would have revolved 336.3 times. Since each 

 revolution consumes four cylinders full of steam, the quantity of steam supplied 

 by the boiler to the cylinders per mile will be found by multiplying the con- 

 tents of the cylinder by four times 336.3, or 1,345.2. 



The cylinders of the Hecla were 12^ inches in diameter, and 18 inches in < 

 length, and consequently their contents were 1.28 cubic feet for each cylin- > 

 der: this being multiplied by 1,345.2 gives 1,721. 86 or 1,722 cubic feet of 

 steam per mile. It appears, therefore, that supposing the priming either 

 nothing or insignificant, which was considered to be the case in these experi 



